Voltage regulators are well known in the art. These devices attempt to provide a stable, nearly constant (regulated) supply voltage to a load. Further, these devices attempt to maintain the supply voltage at the nearly constant value regardless of the current demands of the load. In one practical application, voltage regulators are utilized in complex electronic systems to convert an unregulated supply voltage (e.g., from a battery) into a regulated supply voltage of a predetermined value that is supplied to one or more discrete components of the complex electronic systems.
Complex electronic systems incorporating components such as microprocessors, field programmable gate arrays (FPGAs), and digital application specific integrated circuits (ASICS) often require voltage regulators that provide soft start, voltage tracking, and/or timing delay control of the supplied system voltage. Each of these functions, which are described in additional detail in the following paragraphs, serves to prevent faulty operation and/or damage to the components of the complex electronic system.
The soft-start and soft-stop functions control system voltages at startup and shutdown such that supply voltages rises at a known controlled rate at startup, stop reliably at the programmed operating voltage without overshoot, and then decrease at a controlled rate at shutdown. The soft-start function is particularly used to control inrush currents in capacitors, minimize load surges in battery sources, or to moderate the effect of voltage spikes. The soft-start function typically utilizes a user-supplied external capacitor that is mounted to a dedicated external pin of the voltage regulator. A small current applied to this soft-start capacitor during the startup process causes the charge stored in the capacitor to gradually increase, and this gradually increasing charge is utilized to produce the ramped voltage signal. During soft-stop operations, the current applied to the soft-start capacitor is reversed, and the voltage ramps down as the soft start capacitor discharges.
The voltage tracking function controls system voltages during operation such that they remain equal to or less than a predetermined master supply voltage. The predetermined master supply voltage is typically transmitted to the voltage regulator by way of another external pin (i.e., different from the external pin used to implement the soft start function).
The delay control function regulates system operations by generating a “POWER GOOD” signal at start up that asserts a TRUE value after a programmable delay once predetermined output conditions are met. On shutdown, the delay function maintains an active output power for a programmable delay time after an ENABLE input signal has been removed from an external control source. Similar to the soft start function, the programmable delay is generated using an external user-supplied capacitor that is connected to a dedicated pin.
Conventional systems providing soft start, voltage tracking, and delay control functions are controlled using control signals applied to three separate external pins. Because the number of external pins is limited by the selected package type, it is desirable to minimize the number of external pins needed to perform start up and voltage control functions.
What is needed is a method and structure for controlling both soft-start and voltage control functions in a complex electronic system using a single external pin. In addition, what is needed is a method and structure for allowing a customer complete control of both startup and shutdown supply sequences using a minimum number of external pins.